Does muscle fiber type determine sports performance?

Fast twitch vs Slow twitch

Do you ever wonder why some people are just amazing at what they do? Let’s take Usain Bolt as an example. The guys is fast! Like really fast. So fast that he has at the time of this blog post won 8 Gold Olympic Medals with another one being stripped in the 4 x 100 meter relay because of a teammate who tested positive for methylhexaneamine. Besides that there can be so many different factors and influences in great and successful athletes but we are going to just take a look at one.

Muscles can contain three different types of skeletal muscle twitch fibers. These fibers are categorized as slow twitch, fast twitch, and intermediate twitch. Everyone has these fibers but muscles are made up of different percentages of each type of muscle fiber. You can have more fast twitch than slow twitch fibers and that is one reason why different body types excel at different sports. As a marathon runner you could excel if your muscle composition was mostly made of slow twitch muscle fibers because you would be able to slowly replace energy as it is being used. To understand why this makes a big difference we need to look at what the muscles use for fuel.

The energy needed to contract a muscle directly comes from ATP. ATP’s full name is adenosine triphosphate and this is the energy molecule that the body produces for energy. Along with ATP the other two major ingredients required for muscle contraction are glucose and oxygen. When a muscle is not active it will store oxygen in the muscle cells for whenever you need to use that muscle so it is nearby and ready to go when called upon. Myoglobin is the red protein that is responsible for storing this oxygen. The oxygen stored in your muscles is quickly used up during exercise and your body will replenish these oxygen stores via blood flow bringing more myoglobin proteins to the muscles. The other fuel your body uses for muscle contractions is glucose and it is used to create ATP.

(calcium and magnesium are also pretty important to muscle function but those will be in another post)

Your body is able to replenish ATP stores as long as there is enough glucose in the bloodstream and enough of a glucose reserve stored in your muscle cells and liver. If you have ever experienced a bonk or extreme fatigue during training then most likely you are running out of glucose and are unable to make energy which causes the inability to do anything. These energy and nutrient delivery systems are directly related to the different types of muscle fibers and their structure.jh

Fast Twitch Muscle Fibers

Fast twitch muscle is also known by its other name “white muscle”. The reason they categorize this as white muscle is because of the low amount of myoglobin, fewer mitochondria, and less blood capillaries, which makes the muscle fibers appear lighter in color. So fast twitch fibers have less oxygen stores and less ways to make ATP (energy). Along with that, they have modified contractile proteins that make them very efficient at delivering calcium to the muscle cell which results in a faster contraction. This is great for powerful and explosive movements but as a result of the modifications to the contractile proteins they tire and fatigue a lot faster. You can find a greater concentration of these fibers in athletes that excel at short distance events and fast movement activities.

Slow Twitch Muscle Fibers

Slow twitch muscle fibers are called “red muscle”. These have the opposite makeup compared to fast twitch muscle fibers and contract at a much slower rate. They have a better blood supply, more mitochondria, and a lot more myoglobin which allows them to store a lot more oxygen in the muscle. Because of this slower contraction rate they are able to replenish their ATP energy stores before they can fatigue, thus keeping them operating longer. You can see a high concentration of these slow twitch muscle fibers in long distance athletes.

Intermediate Twitch Muscle Fibers

This is a less known type of muscle fiber but it is exactly what it sounds like. Also known as “pink muscle”, they are the middleman between fast twitch and slow twitch. Like slow twitch muscle they contain more capillaries, myoglobin, and mitochondria but they are not as efficient at replacing their ATP supplies. They are however able to produce more force than the slow twitch muscle which gives them more utility. One theory is that they are fast twitch fibers that have been converted to intermediate twitch fibers by endurance training.

The different makeup of these muscle fibers can have a huge impact on training. Different body types will respond differently to training adaption. This is a major benefit to the highly popular HIIT training by helping to activate and condition these fast twitch muscle fibers. So what decides this makeup of different muscle fiber types? Research has shown that long distance vs short distance training does not have a huge effect on the distribution of these fibers. There is definitely a genetic component to the total amount of different fibers.

Here is an example that shows the fiber distribution of how different fiber types can be distributed through different track and field events. People with these percentages would excel at their given track and field event.